Mario vs. Boo is a game where the player has the choice to choose Mario's side or Boo's side. In Mario's story, Yoshi is trapped by Boo, and Mario is sent to a very far away place. Mario needs to finish his adventure to save Yoshi and defeat Boo. In Boo's story (unlocked after beating Mario's), Mario gets Yoshi back, and Mario uses Boo's cannon to shoot himself back to the Mushroom Kingdom, and Boo needs to get there.